Aerosol Wind Tunnel Research Articles

An efficient strategy to enhance air filtration through the synergistic effects of ultrasonics and seed particles

The individual and combined effects of ultrasonic acoustics agglomeration (AA) with the addition of seeding water droplets for particle agglomeration (DA) are demonstrated for their efficiency in manipulating the number concentration of airborne particles as a pre-treatment mechanism in filtration systems. Through a series of experiments in a purpose-built aerosol wind tunnel with particle separation and filtration stages, it is demonstrated that the combined effect of AA and DA can reduce the particle number concentration by up to 30 % as a result of extensive particle agglomeration. This reduction is significantly higher than that achieved either by the individual action of AA or DA and higher than the direct superposition of the reductions achieved individually by these two actions. In addition, the synergistic effects of combining AA and DA can extend filter operating lifespan significantly by up to 57 % due to lower mass deposition in the filter. A theoretical model is developed for the prediction of filters’ pressure drop patterns under various pre-conditioning methods. The current study sheds light on the development of filtration pre-conditioning systems that are capable of enhancing particle removal efficiencies alongside reduced pressure drop increment across filters, leading to both environmental and cost benefits.

Comprehensive performance evaluation of six bioaerosol samplers based on an aerosol wind tunnel

Choosing a suitable bioaerosol sampler for atmospheric microbial monitoring has been a challenge to researchers interested in environmental microbiology, especially during a pandemic. However, a comprehensive and integrated evaluation method to fully assess bioaerosol sampler performance is still lacking. Herein, we constructed a customized wind tunnel operated at 2–20 km/h wind speed to systematically and efficiently evaluate the performance of six frequently used samplers, where various aerosols, including Arizona test dust, bacterial spores, gram-positive and gram-negative bacteria, phages, and viruses, were generated. After 10 or 60 min of sampling, the physical and biological sampling efficiency and short or long-term sampling capabilities were determined by performing aerodynamic particle size analysis, live microbial culturing, and a qPCR assay. The results showed that AGI-30 and BioSampler impingers have good physical and biological sampling efficiencies for short-term sampling. However, their ability to capture aerosols at low concentrations is restricted. SASS 2300 and BSA-350 wet-wall cyclones had excellent enrichment ratios and high microbial cultivability in both short-term and long-term sampling; however, they were not suitable for quantitative studies of aerosols. Polycarbonate filter samplers showed outstanding performance in physical and long-term sampling but lacked the ability to maintain microbial activity, which can be improved by gelatin filter samplers. However, limitations remain for some fragile microorganisms, such as E. coli phage PhiX174 and coronavirus GX_P2V. In addition, the effects of wind speed and direction should be considered when sampling particles larger than 4 µm. This study provides an improved strategy and guidance for the characterization and selection of a bioaerosol sampler for better measurement and interpretation of collected ambient bioaerosols.

Theoretical and Experimental Evaluation of a Compact Aerosol Wind-tunnel and its Application for Performance Investigation of Particulate Matter Instruments

In this study, we developed a compact low-speed wind tunnel. First, we computationally analysed the flow quality of different wall shapes for the contraction section, the most critical part of a wind tunnel, and selected the design exhibiting minimal boundary layer separation at simulated flow velocities of 2 and 8 km h−1. Then, after constructing the wind tunnel, we experimentally evaluated its overall performance based on different parameters per the United States Environmental Protection Agency (U.S. EPA) guidelines (40 CFR 53.62). The air velocity and turbulence profiles were uniform, displaying ≤ 10% variation in the section we tested. Additionally, we measured the mass concentrations and size distributions of polydisperse dust particles, which were generated by a custom-made rotary dust feeder to ensure the homogeneity of the aerosol, inside the wind tunnel at air velocities of 2 and 8 km h−1 and found ≤ 10% deviation for the mean values across the test section relative to those for the central sampling point. We also assessed the effectiveness of the Well Impactor Ninety-Six (WINS) and Very Sharp Cut Cyclone (VSCC) in the wind tunnel at an air velocity of 8 km h−1 by determining the D50 cutoffs, which, being 2.44 ± 0.05 µm and 2.54 ± 0.05 µm, respectively, fulfilled U.S. EPA’s criteria. Furthermore, we compared the performance of a low-cost sensor against that of a reference instrument in measuring PM2.5 concentrations, and our results agreed with those from previous studies.

Evaluation of a passive method for determining particle penetration through protective clothing materials

ABSTRACTThe risk of workers' exposure to aerosolized particles has increased with the upsurge in the production of engineered nanomaterials. Currently, a whole-body standard test method for measuring particle penetration through protective clothing ensembles is not available. Those available for respirators neglect the most common challenges to ensembles, because they use active vacuum-based filtration, designed to simulate breathing, rather than the positive forces of wind experienced by workers. Thus, a passive method that measures wind-driven particle penetration through ensemble fabric has been developed and evaluated. The apparatus includes a multidomain magnetic passive aerosol sampler housed in a shrouded penetration cell. Performance evaluation was conducted in a recirculation aerosol wind tunnel using paramagnetic Fe3O4 (i.e., iron (II, III) oxide) particles for the challenge aerosol. The particles were collected on a PVC substrate and quantified using a computer-controlled scanning electron microscope. Particle penetration levels were determined by taking the ratio of the particle number collected on the substrate with a fabric (sample) to that without a fabric (control). Results for each fabric obtained by this passive method were compared to previous results from an automated vacuum-based active fractional efficiency tester (TSI 3160), which used sodium chloride particles as the challenge aerosol. Four nonwoven fabrics with a range of thicknesses, porosities, and air permeabilities were evaluated. Smoke tests and flow modeling showed the passive sampler shroud provided smooth (non-turbulent) air flow along the exterior of the sampler, such that disturbance of flow stream lines and distortion of the particle size distribution were reduced. Differences between the active and passive approaches were as high as 5.5-fold for the fabric with the lowest air permeability (0.00067 m/sec-Pa), suggesting the active method overestimated penetration in dense fabrics because the active method draws air at a constant flow rate regardless of the resistance of the test fabric. The passive method indicated greater sensitivity since penetration decreased in response to the increase in permeability.

Development of aerosol wind tunnel and its application for evaluating the performance of ambient PM10 inlets

Size selective particulate matter (PM) sampling inlets play an important role in ambient PM measurement. Improper design of the sampling inlets results in collecting of PM with undesired size, which leads to significant errors in the measurement of ambient PM concentrations. Therefore, the performance of PM inlets should be carefully evaluated in a proper environment prior to their field of applications. In this study, a new aerosol wind tunnel system was designed to evaluate the performance of ambient PM10 inlets and evaluated for the uniformity of wind speed distribution and aerosol concentration. In addition, a custom–made PM10 inlet was tested in the aerosol wind tunnel to determine its 50% cutoff diameter. Results of the wind speed distributions show that the percentage deviations from the mean wind speeds at any measurement point are less than 10% with turbulence intensity of less than 5% for three different wind speed levels (0.57m/s, 2.22m/s, and 6.67m/s). Results from the aerosol concentration measurements show that the percentage deviations from the mean aerosol concentrations at any measurement point are less than 10% for three different wind speed levels, which meets the aerosol wind tunnel performance specifications specified by the U.S. Environmental Protection Agency (EPA). Results from PM10 inlet performance tests show that the 50% cutoff diameters of the PM10 inlet are 10.0 μm, 10.3 μm, and 10.0 μm at wind speeds of 0.57m/s, 2.22m/s, and 6.67m/s, respectively. The PM10 inlet is characterized to meet the performance specifications for PM10 inlets, 10.0±0.5 µm, specified by the U.S. EPA. The results indicate that the newly developed aerosol wind tunnel meets the performance requirements for evaluating the performance of PM10 size selective inlets.

A Multidomain Magnetic Passive Aerosol Sampler: Development and Experimental Evaluation

An innovative “quarter-sized” multidomain magnetic passive aerosol sampler (MPAS) has been developed, mainly for determining particle penetration through personal protective ensembles. The MPAS is a 28 mm disc with a height of 8.6 mm. It consists of 186 small magnets with about 140 on the collection area, arranged in an alternating N and S pole pattern. In contrast to conventional passive samplers, it uses magnetic force to collect a quantifiable amount of surrogate Fe3O4 particles within a substantially shortened sampling time. This article presents detailed design, principles of operation, performance evaluation, and the development of a deposition velocity model for the MPAS. Performance of the MPAS was evaluated under various test conditions, including different particle sizes ranging from 95 to 350 nm and wind speeds ranging from 0.48 to 1.17 m/s. A previously developed recirculation aerosol wind tunnel was employed to evaluate its performance. Experimental results show that the dimensionless deposit...

A Recirculation Aerosol Wind Tunnel for Evaluating Aerosol Samplers and Measuring Particle Penetration through Protective Clothing Materials

A recirculation aerosol wind tunnel was designed to maintain a uniform airflow and stable aerosol size distribution for evaluating aerosol sampler performance and determining particle penetration through protective clothing materials. The oval-shaped wind tunnel was designed to be small enough to fit onto a lab bench, have optimized dimensions for uniformity in wind speed and particle size distributions, sufficient mixing for even distribution of particles, and minimum particle losses. Performance evaluation demonstrates a relatively high level of spatial uniformity, with a coefficient of variation of 1.5-6.2% for wind velocities between 0.4 and 2.8 m s(-1) and, in this range, 0.8-8.5% for particles between 50 and 450 nm. Aerosol concentration stabilized within the first 5-20 min with, approximately, a count median diameter of 135 nm and geometric standard deviation of 2.20. Negligible agglomerate growth and particle loss are suggested. The recirculation design appears to result in unique features as needed for our research.

Evaluation of Nano- and Submicron Particle Penetration through Ten Nonwoven Fabrics Using a Wind-Driven Approach

Existing face mask and respirator test methods draw particles through materials under vacuum to measure particle penetration. However, these filtration-based methods may not simulate conditions under which protective clothing operates in the workplace, where airborne particles are primarily driven by wind and other factors instead of being limited to a downstream vacuum. This study was focused on the design and characterization of a method simulating typical wind-driven conditions for evaluating the performance of materials used in the construction of protective clothing. Ten nonwoven fabrics were selected, and physical properties including fiber diameter, fabric thickness, air permeability, porosity, pore volume, and pore size were determined. Each fabric was sealed flat across the wide opening of a cone-shaped penetration cell that was then housed in a recirculation aerosol wind tunnel. The flow rate naturally driven by wind through the fabric was measured, and the sampling flow rate of the Scanning Mobility Particle Sizer used to measure the downstream particle size distribution and concentrations was then adjusted to minimize filtration effects. Particle penetration levels were measured under different face velocities by the wind-driven method and compared with a filtration-based method using the TSI 3160 automated filter tester. The experimental results show that particle penetration increased with increasing face velocity, and penetration also increased with increasing particle size up to about 300 to 500 nm. Penetrations measured by the wind-driven method were lower than those obtained with the filtration method for most of the fabrics selected, and the relative penetration performances of the fabrics were very different due to the vastly different pore structures.

Evaluation of IOM Personal Sampler at Different Flow Rates

The Institute of Occupational Medicine (IOM) personal sampler is usually operated at a flow rate of 2.0 L/min, the rate at which it was designed and calibrated, for sampling the inhalable mass fraction of airborne particles in occupational environments. In an environment of low aerosol concentrations only small amounts of material are collected, and that may not be sufficient for analysis. Recently, a new sampling pump with a flow rate up to 15 L/min became available for personal samplers, with the potential of operating at higher flow rates. The flow rate of a Leland Legacy sampling pump, which operates at high flow rates, was evaluated and calibrated, and its maximum flow was found to be 10.6 L/min. IOM samplers were placed on a mannequin, and sampling was conducted in a large aerosol wind tunnel at wind speeds of 0.56 and 2.22 m/s. Monodisperse aerosols of oleic acid tagged with sodium fluorescein in the size range of 2 to 100 μ m were used in the test. The IOM samplers were operated at flow rates of 2.0 and 10.6 L/min. Results showed that the IOM samplers mounted in the front of the mannequin had a higher sampling efficiency than those mounted at the side and back, regardless of the wind speed and flow rate. For the wind speed of 0.56 m/s, the direction-averaged (the average value of all orientations facing the wind direction) sampling efficiency of the samplers operated at 2.0 L/min was slightly higher than that of 10.6 L/min. For the wind speed of 2.22 m/s, the sampling efficiencies at both flow rates were similar for particles < 60 μ m. The results also show that the IOM's sampling efficiency at these two different flow rates follows the inhalable mass curve for particles in the size range of 2 to 20 μ m. The test results indicate that the IOM sampler can be used at higher flow rates.

Considerations for Modeling Particle Entrainment into the Wake of a Circular Cylinder

The objective of this work is to evaluate the performance of the steady state Reynolds Averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) models for estimating concentration of low Stokes number aerosols (Stk = O(10−4)) in the wake of a bluff body. These simulations are compared with experimental data. In the simulations and experiments, particles are released upstream of the body and convected downstream, where some are entrained into the wake. The air velocity is computed using a steady state renormalized group k ∼ ϵ model. Lagrangian particle trajectory simulations are performed in conjunction with each airflow model to calculate concentrations. The experiments are performed in an aerosol wind tunnel in which phase Doppler velocimetry measurements are obtained for the velocity field and aerosol concentration. The RANS model yields a wake concentration deficit that extends downstream past x/D = 10, while the experiments produce elevated concentrations immediately downstream of the near wake. It is postulated that the concentration peak is at least in part attributed to particle interaction with the boundary layer by the following mechanism. Particles are transported into the boundary layer by turbulent diffusion, turbophoresis, and/or inertial forces. Particles then separate from the cylinder with the airflow and travel in a sheath around the periphery of the near wake to converge at the downstream edge of the near wake. Underestimation of the wake concentration by the RANS model is potentially due to inadequacy in the boundary layer approximation used in the model.

An Aerosol Wind Tunnel for Evaluation of Massive-Flow Air Samplers and Calibration of Snow White Sampler

Massive-flow air samplers are being deployed around the world to collect aerosol samples for analysis of radioactivity as a result of nuclear tests and nuclear accidents. An aerosol wind tunnel capable of an 1100 m3 min−1 flow rate was built at Lovelace Respiratory Research Institute (LRRI) to test the sampling efficiency of these samplers. This aerosol wind tunnel uses a stationary air blender to enhance mixing, and therefore it achieves the required uniform distribution of wind speed and aerosol concentration in the test section. The test section of the wind tunnel has a cross section that is 4.3 m × 3.7 m. The aerosol wind tunnel was tested for performance in terms of distribution of wind speed, turbulent intensity, SF6 tracer gas concentration, and aerosol concentration. Test criteria consistent with U.S. Environmental Protection Agency (EPA) and American National Standards Institute (ANSI) standards were adopted as the guidelines for the aerosol wind tunnel. Additional criteria for aerosol wind tunne...

Wind Tunnel Evaluation of an Aircraft-Borne Sampling System

The U.S. Environmental Protection Agency (EPA), the Florida Department of Environmental Protection (FLDEP), and Texas A&M University collaborated in the design, construction, and testing of a unique, highly crosslinked, Teflon-coated inlet and manifold gas and aerosol sampling system that is being used in EPA aircraft atmospheric pollution characterization studies. The aircraft-borne ambient sampling system, which consists of a Teflon-coated shrouded probe coupled to a Teflon-coated aluminum manifold, is designed to collect reactive gases (e.g., mercury and halide species) and aerosols for subsequent analysis and characterization. The shrouded inlet probe was tested for particle transmission ratios in a high-speed aerosol wind tunnel. An existing wind tunnel was upgraded from a maximum wind speed of 13.4 m/s (48 km/h or 30 miles/h) to 50.5 m/s (182 km/h or 113 miles/h) to test this probe. The wind tunnel was evaluated for compliance with the criteria of ANSI 13.1 to establish the acceptability of its use in testing probes. The results demonstrated that the velocity and tracer gas concentration profiles were within the specified limits. A wellcharacterized ThermoAndersen Shrouded Probe (Model RF-2-112) was also tested to check tunnel performance and test methodology. The results obtained from these tests are in close agreement with earlier published data. When operated at a sampling flow rate of 90 L/min, the aircraft-borne shrouded probe showed a transmission ratio of about 0.76 at 45 m/s (162 km/h or 100 miles/h) for 10 μ m aerodynamic diameter particles. To improve the transmission ratio of the sampling probe, the sampling flow rate was reduced to 80 L/min and the air speed increased to 50.5 m/s, which increased the transmission ratio to about 0.9 for 10 μ m particles. Further reduction of the flow rate to 60 L/min increased the transmission to 1.2. The Teflon-coated manifold, which is located downstream of the shrouded probe, was statically tested for transmission ratio at flow rates of 90 L/min and 30 L/min. The results were a transmission ratio of about 0.80 for 10 μ m aerodynamic diameter particles. The combination of the shrouded probe operated at 60 L/min with a transmission ratio of 1.2 and the manifold with its transmission of 0.8 will give an overall transmission of about unity for 10 μ m aerodynamic diameter particles at a flight speed of 50.5 m/s. These findings suggest that shrouded probes can be used for low speed (∼ 100 miles/h) aircraft applications. The transmission ratio of these probes is a significant improvement over the conventional aircraft-mounted, sharp-edged isokinetic diffuser-type inlets.

Numerical simulation studies of the turbulent airflow through a shrouded airborne aerosol sampling probe and estimation of the minimum sampler transmission efficiency

In this paper, we present the results of axisymmetric, finite element-based numerical simulation studies of the turbulent airflow through a shrouded airborne aerosol sampling probe operating in a 100 m s −1 airstream at a sampling altitude of 6069 m (20000 ft) above sea level. We calculate the aspiration efficiency of the sampler and the minimum transmission efficiency of the diffuser probe by simulating the trajectories of particles in the size range 0.1–1.5 μm as they pass through the system. We compare these results with wind tunnel measurements and finite difference-based numerical simulation studies. We also estimate the minimum transmission efficiency of a shrouded probe recently tested in an aerosol wind tunnel.

Numerical simulation studies of the turbulent airflow through a shrouded airborne aerosol sampling probe and estimation of the minimum sampler transmission efficiency

In this paper, we present the results of axisymmetric, finite element-based numerical simulation studies of the turbulent airflow through a shrouded airborne aerosol sampling probe operating in a 100 m s −1 airstream at a sampling altitude of 6069 m (20000 ft) above sea level. We calculate the aspiration efficiency of the sampler and the minimum transmission efficiency of the diffuser probe by simulating the trajectories of particles in the size range 0.1–1.5 μm as they pass through the system. We compare these results with wind tunnel measurements and finite difference-based numerical simulation studies. We also estimate the minimum transmission efficiency of a shrouded probe recently tested in an aerosol wind tunnel.

Aerosol penetration through a model transport system: comparison of theory and experiment

Numerical predictions were made of aerosol penetration through a model transport system. A physical model of the system was constructed and tested in an aerosol wind tunnel to obtain comparative data. The system was 26.6 mm in diameter and consisted of an inlet and three straight sections (oriented horizontally, vertically, and at 45{degree}). Particle sizes covered a range in which losses were primarily caused by inertial and gravitational effects (3-25 {mu}m aerodynamic equivalent diameter (AED)). Tests were conducted at two flow rates (70 and 130 l/min) and two inlet orientations (parallel and perpendicular to the free stream). Wind speed was 3 m/s for all test cases. The cut points for aerosol penetration through the experimental model vis-a-vis the numerical results are as follows: At a flow rate of 70 l/min with the inlet at 0{degree}, the experimentally observed cut point was 16.2 {mu}m AED while the numerically predicted value was 18.2 {mu}m AED while the numerically predicted value was 18.2 {mu}m AED. At 130 l/min and 0{degree}, the experimental cut point was 12.8 {mu}m AED as compared with a numerically value of 13.7 {mu}m AED. At 70l/min and a 90{degree}, the experimental cut point was 12.0 {mu}m AED while themore » numerically calculated value was 11.1 {mu}m AED. Slopes of the experimental penetration curves are somewhat steeper than the numerically predicted counterparts.« less

Performance Evaluation of Continuous Air Monitor (CAM) Sampling Heads

Continuous air monitor (CAM) samplers are used to detect radioactive aerosol particles in nuclear facilities and to provide alarm signals should the concentrations exceed a multiple of the derived air concentration (DAC) of the radionuclide of concern in a set amount of time. Aerosol particles are drawn into a CAM sampler where collection is to take place upon a filter. Radioactivity of the particles is determined with a detector that is placed in close proximity to the filter face. An important determinant of CAM performance is the ability of the inlet and body of the CAM to transport particulate matter in the inhalable-size range (less than or equal to 10 microns aerodynamic diameter) to the filter without substantial loss or bias with respect to particulate size. Three types of CAM samplers were tested in a low-velocity aerosol wind tunnel to determine the degree to which particles penetrate through the flow systems to the collection filter under conditions typical of normal room air exchange rates. Two air velocities were used: 0.3 and 1.0 m s-1. The CAM samplers were primarily operated at a flow rate of 56.6 L min-1, although some tests were conducted at a flow rate of 28.3 L min-1. The CAM units were prototypes manufactured by Kurz Instruments, Eberline Instrument Corporation, and Victoreen Inc. These three units represent three different approaches to CAM head design. At an air speed of 1 m s-1, aerosol penetration to the filters of the Kurz unit was essentially 100% for particle sizes of 3 and 7-microns aerodynamic diameter and was 86% for a size of 15 microns. For the Eberline sampler, the penetration was over 80% for 3-microns particles but was reduced to less than 2% for 7-microns particles. The victoreen sampler showed penetration values of 98% for 3-microns aerodynamic diameter particles, 88% for 7-microns particles and 4% for a size of 15 microns. Air speed had little effect on the penetration results for the two speeds which were tested. Tests were conducted to determine the uniformity of deposits on the filters of the CAM samplers. For a particle size of 10 microns, the deposits were nonuniform for all three of the instruments.